Although metastatic involvement of the thyroid is not commonly considered in the differential diagnosis of a patient with a thyroid mass, such involvement may be more frequent than has been previously appreciated. To evaluate the prevalence of this disorder, all patients with thyroid nodules referred in a 12-month period underwent fine-needle aspiration, regardless of history or scan results. Seventy patients were studied, six of whom had a previous history of malignant disease. Two of the latter had coincidental thyroid disease, but four (5.7% of the total series) had carcinoma metastatic to the thyroid gland. Two of these patients had had no evidence of malignancy for years prior to the appearance of the thyroid metastases. In one case, the metastasis was functioning nodule. Fine-needle aspiration cytology provided prompt and accurate diagnosis in all patients. We conclude that (1) metastatic carcinoma to the thyroid is not rare, (2) its presentation may be confused with primary thyroid disease, and (3) fine-needle aspiration cytology is the procedure of choice in such cases, and should be done early in the evaluation of all patients with thyroid nodules, regardless of the presumed etiology or function by scan.